THE rCETAL MEMBRANES OF THE VERTEBRATES. 179 



epochs) without having to look for incipient stages of any of 

 them among our present ichthyopsids. Nay, we may even 

 say that of this earlier, archaic starting-point evident traces 

 have been preserved in the teleostomes, the dipnoi, and the 

 amphibians, so that we have to reconsider most seriously 

 whether it will be wise to go on subdividing the vertebrates 

 into the two subdivisions of those that have and those that 

 have not the foetal envelopes above mentioned. 



Now let us consider the facts as they present themselves to 

 us, when we want to test the question whether one single 

 original foetal envelope could not after all be at the bottom of 

 the three complicated involucra we have just mentioned. As 

 far as I can see, we are only in need of this one assumption, 

 that an invertebrate ancestor was possessed of what we call 

 an exterior larval layer (such as are not uncommon among 

 different worms, and. as we find them, with certain further 

 complications, in some arthropods), to be able to explain how, 

 in their vertebrate descendants, chorion, amnion, and allantois 

 gradually came into being. 



Part of this hypothetical assumption we see actually realised 

 under our eyes wherever one of the mammals goes through 

 its normal stages of development. 



We find that the cell-material out of which the embryo is 

 going to be built up is surrounded by an expanded cell-layer, 

 which takes no part whatever in the composition of the future 

 embryo. Here we actually have our single larval layer that 

 will be stripped off later, and that surrounds what are going 

 to be the formative cells. 



In all mammals it is this very larval layer which will become 

 the outer wall of the blastocyst, what we have above called 

 the chorion. 



But before following it in its further transformations, we 

 have to ask ourselves, what can be the reason that this outer 

 larval layer, this trophoblast, is so far away from the formative 

 cells of the embryo which adhere to it only at one point ? 



We have only to recall the fact of the pilidium larva, in 

 which, similarly, the distance between the outer layer and the 



12 ,S 



